Conventionally, there has been proposed in Japanese Laid Open Patent Application No. 60-256129, a copying apparatus in which an original document on an original document platform made of glass is illuminated by an exposure lamp, and a surface of an electrostatic latent image support member is exposed through a mirror system and a projecting lens assembly by the reflected light. At the side of the projecting lens assembly, there is provided an automatic exposure control device having a directive light quantity detecting device, which is comprised of a photo detector, and a hood for regulating the photo detector to allow the photo detector to receive only the reflected light of the exposure lamp guided through mirror system from the original document. Thus, the density of the original document is measured by detecting the light reflected from the original document, the light quantity of the exposure lamp is controlled according to the density of the original document or condition of colors etc., so as to get a suitable density of a copying image automatically.
However, in the conventional copying apparatus, a second location on the original document from which a light to be measured is reflected coincides with a first location on an original document to be exposed on the light path passing through a center of a projecting lens assembly.
Therefore, the above described apparatus has such a disadvantage that due to the poor response characteristic of an exposure lamp and a time-lag caused by an electric circuit, the result of the detection by the photo detector can not influence the control of a light quantity of the exposure lamp without time-lag.
Consequently, the apparatus has the following disadvantages.
A region of poor image quality is produced on a copy paper, and the width of the region is about 5-6 mm in the direction parallel to the transport direction of a copy paper in the case of equal size magnification. In the case of reduced size magnification, due to the increase of the scanning speed of the exposure lamp, the region is enlarged in inverse proportion to the copying magnification.